Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices, integrated optics, and photomasks. Such processes utilize various energy sources to create a relief image in a film of resist material applied onto a substrate. A positive or negative image of the desired device configuration is first introduced into the resist by exposing it to patterned radiation which induces a chemical change in the exposed portions of the resist. This chemical change is exploited to develop a pattern in the resist, which is then transferred into the substrate underlying the resist.
The efficacy of a lithographic process depends at least in part on the resist used to transfer the pattern into the substrate. Certain types of resists offer particular advantages in the context of specific lithographic processes. For example, solution-developed resists are designed to have absorption characteristics appropriate for use at certain exposure wavelengths. It is axiomatic that, if the resist material is opaque to the exposing radiation, the exposing radiation will not be transmitted into the resist material and the desired chemical change will not occur. Therefore it is important to select a resist material that has the appropriate light transmission characteristics at the wavelength of the exposing radiation. Other considerations that drive the selection of an appropriate resist material include the etch resistance of the resist after it has been exposed and developed.
In this regard, resist materials that contain polymers with ethylenic and/or aromatic unsaturation are typically used in lithographic processes for device fabrication in which the wavelength of the exposing radiation is in the traditional ultraviolet (UV) or deep UV range (i.e., about 240 nm to about 370 nm). These resist materials, however, are often not suitable in processes in which the exposing radiation is 193 nm because the carbon-carbon double bond absorbs radiation at this wavelength. Consequently, resist materials typically used for lithographic processes using wavelengths of 248 nm or more are generally not useful in processes using a wavelength of 193 nm. Because lithographic processes for fabricating devices using 0.18 .mu.m, 0.13 .mu.m, and smaller design rules are likely to use 193 nm-wavelength light as the exposing radiation, resist polymers that do not contain significant amounts of ethylenic unsaturation are sought.
Certain protective groups, when attached to the polymer, function as moieties that render the polymer relatively insoluble in alkaline (basic) solution. In lithographic processes, these moieties are removed upon irradiation and baking of the polymer film in the presence of a radiation-induced acid, and the polymer then becomes relatively more soluble in alkaline solution. After a substantial percentage of the moieties, for example, t-butyl carbonate, t-butyl ester, or t-butyl ether, have been cleaved from the exposed polymer, the polymer in the exposed region of the film is substantially more soluble in an aqueous alkaline developing solution.
The moieties are not cleaved from polymer in the unexposed regions. Therefore, the resist material in those regions is not as soluble in an alkaline solution. If an alkaline solution is used to develop the image projected onto the resist, the material in the exposed regions is dissolved by developer solution while the material in the unexposed regions is not. It is by this mechanism that a positive tone image is developed that corresponds to the image projected into the resist material.
If light is used as the energy source in a lithographic process, the process is referred to as photolithography. If such photolithographic processes utilize an exposure that occurs simultaneously over an entire device or a number of devices being processed on a substrate, the process utilizes what is considered a blanket exposure. A material, i.e., a resist, which is sensitive to the exposing radiation is coated onto a substrate, e.g., a silicon substrate, on which a plurality of devices will be formed. The coating material may be pre-exposure baked and is subjected to spatially discrete radiation, e.g., light that has been passed through a mask so that the light reaching the resist defines a discrete area. The discrete area defines a pattern that is to be transferred onto the underlying substrate either by negative or positive tone. The coated substrate is, if desired, post-exposure baked prior to image development. The resists used in photolithography are referred to as "photoresists."
A blanket exposure is advantageous because it is relatively fast compared to other methods such as the raster scan technique that is employed when the energy used to expose the resist is a beam of electrons. Generally, however, the resolution that is achieved through a blanket exposure with near ultraviolet or visible light is somewhat poorer than that achieved with other methods such as electron beam lithography.
Improved resolution with a blanket exposure can be achieved by using shorter wavelength light such as deep ultraviolet or X-ray light. One approach to a photoresist sensitive to shorter wavelength radiation employs a photoacid generator (PAG) that produces an acid moiety upon irradiation with deep ultraviolet light, together with a polymer that reacts under the influence of heat with the generated acid. Such systems are generally referred to as chemical amplification systems because the production of one molecule of acid by radiation (e.g., light) induces a reaction in a plurality of reactive substituents in the acid-sensitive polymer. Because protective groups are not cleaved from the resist polymer in the unexposed regions, it follows that acid is preferably not generated or otherwise present in the unexposed regions.
Attempts have been made to improve the sensitivity and to reduce the fill shrinkage of chemically amplified resists. By improving the sensitivity of resists, less energy is required to create the image throughout the resist layer. In this regard, resist materials that have been partially deprotected have demonstrated enhanced sensitivity. Partial deprotection means that some, but not all, of the protective groups are cleaved from the polymer prior to use of the polymer in a lithographic process.
Although chemically amplified resists show great promise for fine line resolution, these materials have demonstrated a tendency to shrink and crack after the exposure and post-exposure baking steps of the lithographic process. Shrinkage and cracking occur when the protected polymer is heated in the presence of acid, which releases the protecting groups in the form of gaseous products such as CO.sub.2 and isobutylene from the polymer. Such shrinkage produces a loss of image quality and, in part, counteracts resolution improvement available through use of deep ultraviolet, X-ray, or electron beam exposure. In device fabrication, because of particularly fine design rules, this film shrinkage can significantly affect the quality of the features produced in the lithographic process. Thus, although chemically amplified resists are extremely promising, some improvement is desirable.
One type of resist material that has been suggested as suitable for 193 nm lithographic processes contains a derivatized acrylate or methacrylate copolymer. While resist materials that contain these copolymers demonstrate adequate sensitivity to 193 nm wavelength radiation, the plasma etch resistance of these copolymers does not meet current processing requirements. Therefore, resist materials that are compatible with 193 nm lithographic processes are desired.